In a coherent super-continuum source, ultra short laser radiation pulses having relatively high peak power are typically spectrally broadened by passing the pulses through a short optical fiber which is either tapered to less than single mode dimensions or through a structured optical fiber such as a photonic crystal fiber (PCF), with the PCF being preferred. Typical sources of the laser-radiation pulses are mode-locked titanium sapphire (Ti:sapphire) lasers or Yb-doped mode-locked fiber lasers, both having pulse durations between tens of femtoseconds (fs) and hundreds of fs.
In order to maintain a phase correlation across the broadened spectrum, the typical length of a structured fiber is short, for example between about 5 millimeters (mm) and 10 mm. Depending on a particular application, the spectrally broadened pulses may be used with the entire broadened spectrum, or parts of the broadened spectrum may be tunable selected by a spectrometer or an interference filter. Applications of super-continuum sources include microscopy, spectroscopy and ultrafast amplifier seeding and phase stabilization.
A problem with using tapered or structured fibers for spectral broadening, while maintaining optical coherence across the spectrum, is that a relatively small core-diameter or about 5 micrometers (μm) or less is required. This leads to a poor coupling efficiency into the fiber, for example about 50% or less. In addition, the relatively small core-diameter limits the pulse-energy that can be broadened to about 5 to 10 nanojoules (nJ). At higher energy, damage to the fiber can occur.
Super-continua can also be generated by spectrally broadening ultra-short laser-radiation pulses by focusing the laser-radiation pulses in a bulk optical element of a highly nonlinear material such as Tellurite glass. Such a bulk element is significantly less expensive than a PCF and allows broadening the spectrum of high energy pulses with pulse energies of mJ and more. However, in order to overcome a problem of short interaction length of the focused pulses (resulting from a short Rayleigh range of the focused pulses) in the broadening element, the pulse-energy must be increased to a level where self-focusing effects create what is known as an elongated “filament” of different refractive index in the element. This filament behaves as a waveguide induced in the broadening element, which extends the interaction length of the pulse with the material of the broadening-element.
Filamentation, however, can cause permanent photo modification of the material of the broadening element, if not actual optical damage. This means that the broadening element would need to be periodically “shifted” with respect to a focused beam to expose an undamaged portion of the element to the beam. This would prolong the useful lifetime of the element. Eventually, however, the element would need to be replaced. There is a need for a method of spectral broadening in a bulk nonlinear element that is effective without a need for filamentation.